It is known to employ hot isostatic pressure processing techniques (HIP) for upgrading the mechanical properties of alloys, for example, cast alloys, characterized by the presence of micropores and/or other structural defects. According to U.S. Pat. No. 3,758,347, a metal casting of an alloy based on an element selected from the group consisting of Ni, Co, Fe, and Ti and having internal discontinuities, such as porosity, microfissures, cracks, and the like, can be improved by applying isostatic pressure to the casting at an elevated temperature less than that temperature which will cause substantial degradation of the mechanical properties of the alloy for a time sufficient to close the pores and effect diffusion bonding of the walls of the pores, fissures, etc. Superalloys are mentioned in particular, such as age-hardenable nickel-base superalloys designated by the trademarks Rene 80, Rene 100, etc. Rene 80 contains 0.17% C, 14% Cr, 5% Ti, 0.015% B, 3% Al, 4% W, 4% Mo, 9.5% Co, 0.03% Zr, and the balance nickel, while Rene 100 contains 0.17 % C, 9.5% Cr, 4.2% Ti, 0.015% B, 5.5% Al, 3% Mo, 15% Co, 0.06% Zr, 1% V, and the balance nickel.
According to the patent, in the treatment of Rene 80 castings in an autoclave heated to 2225.degree. F. (1218.degree. C.) at a pressure of 10,000 psig, samples of the alloy were held for about 8 hours and then removed after cooling. The HIP treated samples were compared to samples not given the HIP treatment and following heat treatment. Both the HIP treated and untreated samples were subjected to a solution treatment at 2225.degree. F. (1218.degree. C.) for 2 hours in a vacuum, then inert gas quenched to room temperature followed by heating at 2000.degree. F. (1093.degree. C.) for 4 hours in vacuum and inert gas quenching to room temperature. Following the latter quench, the alloy samples were aged at 1925.degree. F. (1052.degree. C.) for 4 hours, furnace cooled to 1200.degree. F. (649.degree. C.) and held for 1 hour prior to air cooling to room temperature. Finally the two types of samples were heated at 1550.degree. F. (843.degree. C.) for 16 hours in Argon and then cooled to room temperature.
The alloy samples were then tested for stress-rupture at 1600.degree. F. (871.degree. C.) under a stress of 45,000 psi. The results showed that the untreated samples (2 tests) exhibited an average life of about 41.5 hours and an average percent elongation of about 2.5 hours.
The samples treated by HIP (6 samples) exhibited an average stress-rupture value of 141 hours and an average percent elongation of about 11.5%.
As will be apparent, the HIP treatment applied to the aforementioned nickel-base alloy markedly improved the stress-rupture properties.
Elimination of casting defects by using HIP is disclosed in a paper entitled "Elimination of Casting Defects Using HIP" by G. E. Wasielewski and N. R. Lindblad; Proceedings on The Second International Conference on Superalloys--Processing; Seven Springs, Pa., September 1972.
According to the aforementioned paper, stress-rupture properties and room temperature ductility of nickel-base superalloys, for example, alloys referred to by the designation IN-738, Rene 77, IN-792, etc., can be improved by means of the HIP processing technique at temperatures ranging from about 2000.degree. F. (1093.degree. C.) to 2200.degree. F. (1204.degree. C.) for 1 to 10 hours at pressures ranging from about 5,000 to 30,000 psi, a temperature of 2150.degree. F. (1177.degree. C.) to 2200.degree. F. (1204.degree. C.) being particularly preferred to provide 100% densification of the alloy part.
Similar improvements are indicated with HIP processing in a paper entitled "Improved Components Through Howmet's HIP Process", by T. H. Smith and L. Dardi; published in Casting About, Spring (April) 1974 by Howmet Turbine Components Corporation.
In a patent which issued on Nov. 14, 1978 (U.S. Pat. No. 4,125,417), a HIP process is disclosed for use in the same manner for the same purpose as stated above, except that it is applied for salvaging and restoring useful properties of used alloy parts containing such defects as grain boundary voids or dislocations induced by high temperature creep in service, in addition to such cast defects as micropores. Following HIP processing, the alloy part is then subjected to heat treatment (solution treatment and aging) to restore the mechanical properties to their original values.
The concept of employing the HIP process for upgrading the mechanical properties of magnesium and aluminum die castings is disclosed in U.S. Pat. No. 3,732,128, wherein the die casting is subjected to heat and pressure in a container at 300.degree. C. to 600.degree. C. under a pressure of 100 to 10,000 psi for 1 to 72 hours and rapidly cooled while still maintaining the applied pressure. The treated casting is thereafter aged at 100.degree. C. to 250.degree. C. for 1 to 72 hours at atmospheric pressure to improve the mechanical strength of the alloy.
Thus, it is known that the use of HIP processing, involving the simultaneous application of heat and high pressure to investment cast superalloys, results in significant improvements in high temperature mechanical properties which have made it possible for gas turbine designers to specify premium quality castings for critical industrial gas turbine applications. The motivation for using investment castings stems from an industry-wide effort to improve substantially the efficiency and cost effectiveness of gas turbines. In recent years, this effort has been further accentuated by worldwide inflation and a growing shortage of fossil fuel supplies.
It would be desirable to improve still further the capabilities of age-hardenable alloys, e.g., cast superalloys, in light of the ever-increasing high temperature demands being specified for jet engine components, such as for turbine blades employed in the hot end of the engine.